The present invention relates, in general, to a spindle shaft attachment technique for a disk drive. More particularly, the present invention relates to a technique for engagement of the spindle shaft with a relatively planar head disk assembly ("HDA") cover to allow for an increased spindle length and concomitantly greater bearing spacing to establish a relatively higher frequency spindle rocking mode for a given height form factor drive.
Disk drives are computer mass storage devices from which data may be read and/or to which such data may be written. In general, they comprise one or more randomly accessible rotating storage media, or disks, on which data is encoded by various means. In magnetic disk drives, data is encoded as bits of information comprising magnetic field reversals grouped in tracks on the magnetically-hard surface of the rotating disks. When multiple disks, or "platters" are incorporated in a given drive to increase overall storage capacity for a particular form factor, the disks are concentrically stacked in a generally parallel and spaced-apart relationship and affixed at their inner diameter ("ID") to a common hub which is rotationally coupled to a stationary spindle shaft by a pair of bearings.
In addition, a number of read/write heads are mounted on individual access arms interleaved with the individual disks which may be ganged together and attached to a common voice coil motor ("VCM") actuator rotationally coupled to an actuator spindle shaft. The VCM is capable of moving the combined head/arm assembly across the upper and lower disk surfaces at very high speeds to perform seek operations in order to read and/or write data to selected tracks and data sectors on the disks. Utilizing either dedicated, embedded or dedicated plus embedded servo techniques, the disk drive servo system accurately positions the read/write heads with respect to the individual data tracks.
Certain disk drive designs may incorporate cantilever designs for the actuator and disk stack in which the spindles are affixed to only the HDA baseplate. However, most high performance drives employ a design in which the spindle is attached to both the baseplate and cover in order to increase stiffness and improve runout characteristics. Although certain drives have utilized a conically shaped upper spindle tip to engage a corresponding fixed recessed cone machined in the HDA cover to provide the desired spindle stiffness, in most drives the spindles are secured to both the baseplate and cover with screws even though such fasteners may present assembly problems with torque control and stripping of the spindle shaft threads. Regardless, with either technique, tight manufacturing and assembly tolerances must be maintained in order to ensure that the perpendicularity of the actuator and disk stack spindles established by the drive baseplate is not altered by the fixed recessed cone or screws securing the upper end of the respective spindles as the cover is assembled to the baseplate.
With the growing trend toward even lower height form factor disk drives, the height of the screw heads in a typical drive also becomes a significant consideration in meeting drive height constraints. As a result the screws securing the upper end of the actuator and disk stack spindles must be recessed so as not to protrude above the plane of the drive cover. However, recessing the screw heads for a given drive height means that a shorter spindle must be utilized to accommodate the recessed upper screw with a concomitantly shorter spacing available between the upper and lower bearings supporting the hub or actuator on the spindle. Inasmuch as bearings have inherently limited stiffness themselves and exhibit many spring-like properties, shorter spacing between the upper and lower bearings results in reduced stiffness and a reduced rocking mode frequency.
Given the various excitation frequencies in an operating disk drive, whether due to defects and imperfections in the races and ball bearings or other factors, a lower rocking mode frequency can cause drive failure if it becomes coincident with these excitation frequencies, typically on the order of 500 Hz or less. On the other hand, if a relatively higher rocking mode frequency can be established, the problems of its coincidence with the drive excitation frequencies can be avoided.